1. Field of the Invention
The present invention relates to a defrosting control system for use in a heat-pump air-conditioner for air-conditioning an electric vehicle with a compressor that is electrically actuated by the electric energy stored in a battery on the electric vehicle.
2. Description of the Related Art
Generally, heat-pump air-conditioners are used on electric vehicles which are not equipped with a heat source such as an internal combustion engine. The heat-pump air conditioner controls the temperature of the cabin of an electric vehicle through a heat exchange between the thermal energy outside of the cabin and the thermal energy in the cabin using an external heat exchanger located outside of the cabin and an internal heat exchanger located in the cabin. When the heat-pump air conditioner operates in a warming mode to warm the cabin at the time the temperature outside of the cabin is low, the external heat exchanger often tends to become frosted. As the frost on the external heat exchanger progressively increases while the heat-pump air conditioner is in operation, the ability of the external heat exchanger to absorb heat is lowered, and hence the ability of the heat-pump air conditioner to warm the cabin is reduced, with the result that the cabin cannot be warmed as desired.
One solution is to switch the heat-pump air conditioner into a cooling mode to cool the cabin when the external heat exchanger is frosted. In this cooling mode, the external heat exchanger functions as a heat radiator to defrost itself. According to another proposal, an electric heater added to the external heat exchanger is energized to defrost the external heat exchanger when the external heat exchanger is frosted. These two schemes may be combined to defrost the external heat exchanger in a shorter period of time.
The former process is disadvantageous in that while the heat-pump air conditioner is operating in the cooling mode, a comfortable environment cannot be established in the cabin because the cabin is not warmed. The electric heater also poses problems because it consumes a large amount of electric energy, which is not desirable as the consumption of the electric energy stored in battery on the electric vehicle should be minimized, and also because the electric heater increases the cost of the electric vehicle.